In the ink-jet recording method, small droplets of an ink composition are ejected and attached to a recording medium, such as paper, to effect printing. The ink-jet recording method has such a characteristic feature that an image having high resolution with high quality can be printed at a high speed. The ink composition used in the ink-jet recording method generally contains an aqueous solvent as a major component and also contains a colorant component and a moistening agent, such as glycerin, for preventing clogging.
In the case where the ink-jet recording method is applied to paper and fabrics to which the aqueous ink composition is difficult to penetrate, or applied to metal and plastic materials to which the aqueous ink composition does not penetrate, for example, a plate or a film made of a resin, such as a phenol resin, a melamine resin, a vinyl chloride resin, an acrylic resin and a polycarbonate resin, it is required that the ink composition or a reaction liquid contains a component capable of stably fixing the colorant component to the recording medium.
According to the requirement, an ultraviolet ray-curable ink for ink-jet recording containing a colorant, an ultraviolet curing agent (i.e., a polymerizable compound) and a polymerization (photopolymerization) initiator is disclosed (for example, in Patent Document 1). There has been stated that the ink is prevented from causing blur on a recording medium to improve image quality. The ultraviolet ray-curable ink disclosed in Patent Document 1 is an one-component ink composition.
However, in a step of curing a one-component ultraviolet ray-curable ink composition by irradiating with an ultraviolet ray after attaching the ink composition to a recording medium, such a phenomenon is often observed that the only outside (surface) of the ink coated film is cured, but the interior thereof remains as a liquid uncured. This is because an ultraviolet ray is absorbed only by the surface of the coated film but fails to reach the interior thereof. There are some cases on the other hand that only the interior is cured, but the surface is not cured. This is a phenomenon that is often observed when the polymerization rate is low, and it is considered that this is because of polymerization inhibition due to permeation of oxygen from the surface.
In order to avoid the problems, such a measure is often employed that plural kinds of photopolymerization initiators are used in combination. Specifically, a photopolymerization initiator having a high initiator efficiency (which is suitable for surface curing and can suppress polymerization inhibition due to permeation of oxygen) and a photopolymerization initiator suitable for interior curing having a photobleaching effect (in which after cleaving the photopolymerization initiator by absorbing light, the residue of the initiator after decomposition does not absorb an ultraviolet ray to fail to prevent an ultraviolet ray from penetrating into the interior) are used in combination.
Patent Document 1; U.S. Pat. No. 5,623,001
Even in the case where the plural photopolymerization initiators are combined, however, where are some cases where an ultraviolet ray is liable to be absorbed, reflected or scattered by a colorant when a one-component ink composition is coated to a thick film on a recording medium, or an ink composition having a high colorant content is coated. Accordingly, an ultraviolet ray is difficult to penetrate into the interior of the ink coated film to prevent polymerization reaction from proceeding, which brings about insufficient curing. In the case where the addition amount of the photopolymerization initiator is increased therefore, there arises such a vicious cycle that an ultraviolet ray is excessively absorbed in the ink coated film to prevent an ultraviolet ray from penetrating into the interior, which brings about curing failure.
In the invention, the problems are solved by using two or more kinds of ultraviolet ray-curable ink compositions in combination. Specifically, one of the ink compositions contains a photopolymerization initiator suitable for interior curing having a photobleaching effect (in which after cleaving the photopolymerization initiator by absorbing light, the residue of the initiator after decomposition does not absorb an ultraviolet ray to fail to prevent an ultraviolet ray from penetrating into the interior) in a large amount, and the other ink composition contains a photopolymerization initiator having a high initiator efficiency (which is suitable for surface curing and can suppress polymerization inhibition due to permeation of oxygen) and a polymerization accelerator containing fine particles having polymerizable functional groups on the surface thereof. The use of the ink compositions in combination solves the aforementioned problems of curing failure.